Magnetic induction tomography (MIT) is a non-invasive imaging technique with applications in industry and medical imaging. In contrast to other electrical imaging techniques, MIT does not require direct contact of the sensors with the object to be imaged.
MIT applies a magnetic field from one or more generator coils (also called excitation coils) to induce eddy currents in the object (i.e. material) to be studied. In other words, the scanning region is excited with a time-varying magnetic field. The presence of conductive and/or permeable material distorts the energizing field within. The perturbation of said primary magnetic field, i.e. the secondary magnetic field resulting from the eddy currents, is detected by a number of sensor coils (also called measurement coils, detection coils or receiving coils). Sets of measurements are taken and used to recover the position, shape and electromagnetic properties of the object. MIT is sensitive to all of the three passive electromagnetic properties: electrical conductivity, permittivity and magnetic permeability. As a result, for example, the conductivity contribution in a target object can be reconstructed. Because of the magnetic permeability value μR≈1 of biological tissue, MIT is particularly suitable for examination of such tissue.
Prior-art patent application WO2007072343 discloses a magnetic induction tomography system for studying the electromagnetic properties of an object, the system comprising: one or more generator coils adapted to generate a primary magnetic field, said primary magnetic field inducing an eddy current in the object, one or more sensor coils adapted to sense a secondary magnetic field, said secondary magnetic field being generated as a result of said eddy current, and means for providing a relative movement between one or more generator coils and/or one or more sensor coils, on the one hand, and the object to be studied, on the other hand. By moving the generator coil or coils and/or the sensor coil or coils with respect to the target object, the number of independent measurements is increased without more coils being needed. As a result, the sensitivity matrix can be inverted more easily, the solution is more stable, and the reconstructed image has a higher spatial resolution.
The MIT system finds a major application in the field of bio-medical monitoring. The system is required to work for a long time in this case. The system offset, especially the offset caused by temperature, may affect the accuracy of the measurement. The electronic devices normally have a different phase-delay behavior in different temperature conditions, and the mechanical structure will also change when the temperature changes. All of these changes may affect the system accuracy that is required to measure within the accuracy range of milli-degrees. For this reason, the imaging system must be calibrated from time to time. In the conventional calibration method, the patient must be removed from the measurement chamber. Obviously, it is not convenient to carry out this kind of calibration when a seriously injured patient is being monitored and any movement of the patient will exacerbate his or her condition.
There is therefore a need to provide a method and device for calibrating an imaging system without movement of the monitored patient, thus improving the system accuracy.